Transcript Half life and its uses

Half life and its uses
We are going to start with 2 coins each and throw them all and
then count how many had landed with heads facing up and record
this. We will then remove those coins that have landed heads up
and throw the remaining coins. How many of these will land heads
up? Can you predict what would happen to an individual coin?
No of coins
Start
Throw 1
Throw 2
Number
of coins
Throw 3
Throw 4
Throw 5
Throw 6
Number of throws
Throw 7
Half-Life
The half life is the time taken for half of the
radioactive nuclei present to decay.
Time zero: Number of nuclei, N
After one half-life
Time 1: Number of nuclei, N/2
After two half-lives
Number of nuclei, N/4
After three half-lives
Number of nuclei, N/8
After four half-lives
Number of nuclei, N/16
After five half-lives
Number of nuclei, N/32
After five half-lives
Key points
 Each nucleus in a sample of an isotope
decays at random, regardless of what the
other nuclei are doing.
 Radioactive decay is not effected by:
-Chemical state of the isotope, i.e. whether its
in a compound or as its element
-Temperature or pressure
-Whether it is solid, liquid, gas or in solution
Key points
 Half life is: ‘time taken for half the
radioactive nuclei to decay’.
 It is the same regardless of how many
radioactive nuclei we start with.
 The sample never completely disappears, it
just halves each time, this is why radioactive
isotopes can still be detected today, 4 billion
years after the earth was formed.
Examples
Isotope
Half-life
Uranium-238
4.5 x 109 years
Carbon-14
5.7 x 103 years
Strontium-90
28 years
Iodine-131
8.1 days
Bismuth-214
19.7 minutes
Polonium-214
1.5 x 10-4 seconds
How to calculate half life
How to calculate half life
How to calculate half life
Half Life
Half Life
The _________
_______ over
radioactivity of a sample always decreases
alpha ______
beta
time. Each time a decay happens ______,
gamma radiation is emitted. This means a
or ______
nucleus has decayed.
radioactive _______
The problem with trying to measure the time for all
the atoms to decay is that the activity never
zero
reaches _____.
The half life is the _____
time taken for ____
half of the
nuclei now present to decay.
radioactive ______
short half life decays more
An isotope with a ____
quickly than an isotope with a _____
long half life.
Uses of radioisotopes in geology
Radioactive
parent isotope
Potassium-40
Stable daughter
isotope
Argon-40
Half-life
(years)
1.3 x 109
Rubidium-87
Strontium-87
4.5 x 1010
Thorium-232
Lead-208
1.4 x 1010
Uranium-235
Lead-207
7.0 x 108
Uranium-238
Lead-206
4.5 x 109
Radioactive Dating
 C-14 makes up 1/10,000,000 of the carbon in the air.
This same proportion is found in living things. When they
die the C-14 is trapped inside and gradually decays with a
half life of 5600 yrs
 By measuring the proportion of C-14 left and knowing
the half life you can calculate how long ago something was
living.
PRACTICE QUESTION:
An axe handle was found to contain 1 part in 40,000,000 C14. How old is the axe?
The C-14 was originally 1 part in 10,000,000. After 1 half
life it would be down to 1 part in 20,000,000 and after 2 half
lives it would be down to 1 part in 40,000,000. So the axe
handle is 2 half lives old, i.e. 2 x 5600 years = 11,200
years.
Radioactive Dating Question
The half life of C-14 is 5,600 years and C-14 makes
up 1 part in 10,000,000 of the carbon in the air.
Calculate how long ago each of the following was
living material:
a. A fossil containing 1 part in 320,000,000 C-14
b. A spear handle containing 1 part in
80,000,000 C-14
c. An axe handle containing 1 part in
20,000,000 C-14
Remember…
 For these radioisotopes to work as
geological clocks the following must be met:
-Half-life must be known accurately
-No movement of parent or daughter isotopes
out of or into mineral since crystallisation
-no resetting of geological clock through
heating and deformation of the rocks
(metamorphism)
Dating rocks
The proportions of the parent radioisotope
potassium-40 and its daughter decay product
argon-40 can also be used to date igneous
rocks from which the gaseous argon has been
unable to escape.
Dating Rocks
Igneous rocks can be dated if you measure
the ratios of uranium-235 and its decay
product lead. The half life of uranium-235 is
4.5 billion years. Assuming no lead was
present when the rock was formed find the
ages of the rocks:
a) uranium: lead 1:1
b) uranium: lead 75:525
c) uranium: lead 1:0
d) uranium: lead 75:225
Uses of Carbon-14 dating
 Read the green boxes on pages 9-10 for
more examples of where carbon-14 dating
has been used.
Radioactive tracers
 As we have learnt more about isotopes a new field
of medicine has begun to develop, ‘Nuclear
Medicine’.
 Radioisotopes can now be injected into the body
to enable detection and treatment for a range of
problems.
 The half-life of the isotopes is important so as not
to cause unnecessary damage to the cells.
 Examples include iodine-131 to detect thyroid
problems and technetium-99 to detect tumours.